Aberrant regulation of megakaryocyte development is a feature of both essential thrombocythemia (ET) and primary myelofibrosis (PMF). Under normal conditions, committed megakaryocyte progenitors proliferate to a limited extent and then give rise to small numbers of differentiated and polyploid megakaryocytes. However, upon acquisition of mutations in key signaling molecules, such as MPL or JAK2, megakaryocyte progenitors expand and lead to thrombocytosis in ET or myelofibrosis in PMF. The specific molecular changes and mechanisms responsible for the extreme differences in the megakaryocyte phenotype of the two disorders are unknown. In this project, we will identify transcriptional pathways that are dysregulated in PMF megakaryocytes and characterize the causes of aberrant megakaryopoiesis as compared to ET megakaryocytes. We will also determine whether small molecule inducers of megakaryocyte differentiation and polyploidization are effective at restraining the proliferation of aberrant megakaryocytes in MPNs. Finally, we will study the mechanism by which these compounds lead to differentiation and polyploidization of abnormal megakaryocytes. Our overall hypothesis is that megakaryocytes in PMF are abnormal because they aberrantly express myeloid transcription factors and that this program can be reversed with small molecule inducers of megakaryocyte polyploidization and differentiation. This work is innovative in that we are the first to comprehensively describe the differences between PMF and normal megakaryocytes at the molecular level. Moreover, we are using innovative small molecules to advance our understanding of MPNs and to develop new targeted therapies. Our work is significant in that none of the JAK2 inhibitors in clinical trials ameliorate bone marrow myelofibrosis in patients: our research aimed at identifying the root cause of this debilitating condition will aid in development of new therapies.